Labour has questioned why taxpayers are paying more than £170,000 a year for Liam Fox's three official advisers when he appears to prefer to rely on the advice of his best man and travel companion, Adam Werritty.

Two of Fox's highly trained official special advisers are paid £60,740 a year, while a third collects at least £52,215, giving a total wage bill of at least £173,695 a year.

The bill for Fox's advisers is the fourth-highest of any minister in parliament, behind only the prime minister, the deputy prime minister and the chancellor. Fox spends more on advisers than William Hague, the foreign secretary, and the leader of the Lords, Lord Strathclyde.

But the defence secretary appeared to require the services of Werritty, who is not a government employee yet handed out a business card that described him as an "adviser to the Rt Hon Dr Fox MP".

Fox has met Werritty 18 times during overseas trips, where he has met heads of state, ambassadors and dined with General John Allen of US Central Command at a steakhouse in Tampa, Florida. The pair have also met at the Ministry of Defence in Whitehall 22 times over the past 16 months.

Kevan Jones, Labour's defence spokesman, said: "Each day the questions mount up for Liam Fox. He has to explain why he needs Adam Werritty, who appears to be an unofficial adviser, when he has three special advisers costing the taxpayer pay almost £200,000.

"Adam Werritty has been travelling the world giving the impression he was an official adviser and it would appear Dr Fox has not counteracted this impression. We need answers."

Another Labour source said: "Given the extensive connections we are aware exist between Mr Fox and Mr Werritty it is simply ridiculous to suggest he was not an adviser and it is inconceivable that Liam Fox did not know what Adam Werritty was up to."